Growers continually attempt to grow the most productive crops possible in order to maximize yields. Plant growth regulators are one tool that growers can use in order to influence the growth of their plants based on the restrictions of water and temperature. The effects of plant growth regulators on plants under different conditions can vary widely. Further, predicting the effect that application of more than one plant growth regulator simultaneously applied to the plant is difficult.
(S)-abscisic acid (“(S)-ABA”) is an endogenous corn plant growth regulator with many roles in growth and development. For example, (S)-ABA inhibits seed germination, thus antagonizing gibberellins, which stimulate the germination of grains. (S)-ABA promotes stress tolerance and maintains growth under stress conditions (see Sharp R E et al., Root growth maintenance during water deficits: physiology to functional genomics, J Exp Bot, 2004 November, 55(407), 2343-2351). Interestingly, several studies have shown that maintaining ‘normal’ ABA levels in well-watered plants is required to maintain shoot growth in tomato (Sharp R E et al., Endogenous ABA maintains shoot growth in tomato independently of effects on plant water balance: evidence for an interaction with ethylene, J Exp Bot, 2000 September, 51(350), 1575-1584) and Arabidopsis (LeNoble M E et al., Maintenance of shoot growth by endogenous ABA: genetic assessment of the involvement of ethylene suppression, J Exp Bot, 2004 January, 55(395), 237-245). Moreover, (S)-ABA is responsible for the development and maintenance of dormancy in seeds and woody plants, which when deficient in ABA often demonstrate pre-harvest sprouting of seeds due to a lack of dormancy induction.
Further, applications of (S)-ABA have also been shown to provide protection from chilling and drought, as well as to increase the red color of seedless table grapes. Examples of effective commercially available (S)-ABA formulations include ProTone™ and Contego™ (available from Valent BioSciences LLC).
Glycine betaine (“GB”) is a solute that accumulates in plants, micro-organisms and fungi in response to abiotic stress. Among the major cereal crops, only rice does not naturally accumulate GB (Shirasawa K. et al., Accumulation of glycine betaine in rice plants that overexpress choline monooxygenase from spinach and evaluation of their tolerance to abiotic stress, Ann Bot, 2006 September, 98(3), 565-571). Overexpression of bacterial or plant genes in rice to produce GB resulted in low accumulation of GB, but conferred stress tolerance. Exogenous application of GB to plants has been shown to confer abiotic stress tolerance (Chen and Murata, Glycine betaine: an effective protectant against abiotic stress in plants, Trends Plant Sci, 2008 September, 13(9), 499-505). This includes stress resistance inducing resistance to chilling, freezing, and drought across multiple plant species. However, the levels of stress protection observed, although significant, do not reach commercially acceptable thresholds.
Accordingly, there is a need in the art for new methods to improve the growth of plants under abiotic stress conditions.